1. Field of the Invention
The present invention relates to a burst signal detection circuit for detecting the arrival of a burst-like signal, and in particular to a burst signal detection circuit used most suitably for a transmission system for transmitting a high-speed optical burst signal in a system such as a passive optical network (PON) system used in a transmission for optical subscriber system.
2. Description of the Related Art
In the transmission of an optical burst signal, the timing of sending out the burst signal to be transmitted is not definitely determined, and therefore a burst signal detecting signal is required to notify the arrival of the burst signal to the transmission system.
For configuring a flexible network system in which each terminal unit for sending out the burst signal can be accommodated at an arbitrary position on the network and a restriction on the transmission distance is relaxed, it is necessary to accurately discriminate the arrival of a weak burst signal from a remote terminal unit on the one hand and the absence of a burst signal on the other hand.
The present invention relates to a burst signal detection circuit for detecting the arrival of a burst signal at a low level accurately without deteriorating the receiving characteristic. In the prior art, the receiving characteristic may be deteriorated by the DC level fluctuation due to the low-frequency response of a photo-diode (PD) for detecting an optical signal.
FIG. 21 shows a configuration of a conventional burst signal detection circuit in a receiver for receiving an optical burst signal. The optical burst signal detection circuit includes a photo-diode (PD) 210 for converting an optical signal into a current signal, a preamplifier 211 for converting the current signal output from the photo-diode (PD) 210 into a voltage signal, a signal amplifier 213 for amplifying a weak signal output from the preamplifier 211 and producing a sufficiently large logic signal, and an amplitude identifying circuit 215 for detecting the presence or absence of a burst signal based on the signal output from the preamplifier 211.
To process the burst signal, the signal amplifier 213 includes an automatic threshold control (ATC) circuit 214 and a buffer amplifier 213A. The input signal from the preamplifier 211 and a threshold level from the automatic threshold control (ATC) circuit 214 are differentially amplified by the buffer amplifier 213A thereby to output a received signal.
The automatic threshold control (ATC) circuit 214 includes a peak detection circuit 214A, a bottom detection circuit 214B, and a voltage dividing circuit 214C. Upon application of a burst signal thereto through the preamplifier 211, the peak detection circuit 214A and the bottom detection circuit 214B instantaneously detect the maximum level and the minimum level, respectively, of the input signal, and the voltage-dividing circuit 214C sets the central level of the amplitude of the input signal providing the voltage-dividing level as a threshold level and outputs it to the buffer amplifier 213A.
In the amplitude identifying circuit 215, a peak detection circuit 216A in an amplitude detection circuit 216 detects the peak level of the input signal from the preamplifier 211, and a comparator 215A compares the input peak level with a threshold level and outputs a burst signal detection signal indicating the presence or absence of the burst signal.
As shown in FIG. 23A, the frequency characteristic of the conversion efficiency of a photodiode (PD) used as an element for receiving the optical signal is known to have a “drop” in the range of several to several hundred kHz and the conversion efficiency increases in the low-frequency region.
This phenomenon is considered to stem from the carriers generated by the optical signal entering the regions other than the light detecting surface of the photo-diode (PD) where an electric field is not applied. The carriers, to which the electric field is not applied, move slowly, by diffusion, and therefore have a very large time constant.
The shoulder of the low-frequency response characteristic is very small and, usually, can be ignored. In the case where burst signals having considerably different amplitudes are input continuously to the photodiode, however, the problem described below arises.
Assuming that, as shown in FIG. 23B, a second burst signal (packet #2) of a small amplitude arrives and is received by the photo-diode (PD) immediately after the end of a first burst signal (packet #1) of a large amplitude at an interval of a short guide time TG, the current output of the photo-diode (PD) responds to a waveform such that, as shown in FIG. 23C, the DC level (bottom level) thereof gradually rises due to the low frequency response at the time of receiving the first burst signal (packet #1) having a large amplitude and the DC level is restored to the original level at the immediately subsequent time after the first burst signal when the second burst signal (packet #2) is received.
The adverse effect of the DC level variation due to the low frequency response depends on the magnitude of the amplitude of the burst signal. That is, the DC level variation is relatively small for the first burst signal (packet #1) having a large amplitude and therefore has no effect on the detection of the burst signal. However, the DC level variation is large and is not negligible for the second burst signal (packet #2) having a small amplitude, and has an adverse effect on the burst signal detection.
FIGS. 22A and 22B show operating waveforms for detecting the burst signal at the time of the leading portion of the second burst signal (packet #2). FIG. 22A shows an input signal (a) of the amplitude identifying circuit 215, a peak detection output (b) of the peak detection circuit 216A, and a threshold level (c) of the threshold level control circuit 217B.
The send-out time length of a burst signal is predetermined. A reset signal is generated during the guard time TG at predetermined timing, and the peak level of the peak detection circuit 216A is reset by this reset signal.
The input signal (a) shown in FIG. 22A is such that the bottom level output from the photo-diode (PD) is increased by the DC component of the burst signal (packet #1) of a large amplitude that has arrived immediately before. The bottom level of the input signal (a) thus had already exceeded the threshold level (c) at the time of the reset described above. AS shown in FIG. 22B, therefore, the comparator 215B outputs a detection signal (d) erroneously indicating the presence of a burst signal even in the absence of the burst signal.
Specifically, the problem arises in the case where the waveform moves to the low voltage side due to the variation of the bottom level. That is, in this case, while the bottom detection circuit can follow the movement of the waveform, however, the peak detection circuit holds the maximum level of the signal, so that it erroneously detects the amplitude of the input signal as a large amount larger than the actual amplitude. Even in the case where the actual signal is smaller than a specified threshold level or, even worse, in the absence of a signal, a detection signal indicating the presence of the burst signal is erroneously output.